Surface treatment of mild steel



United States Patent 3,228,807 SURFACE TREATMENT OF MILD STEEL Eric Mitchell, Warwick, and Cyril Dawes, Wednesbury, England, assignors to Joseph Lucas (Industries) Limited, Birmingham, England No Drawing. Filed July 2, 1964, Ser. No. 380,026 2 Ciaims. (Cl. 148-121) This invention is a continuation-in-part of our application No. 214,819 filed August 6, 1962, now abandoned and entitled Surface Treatment of Unalloyed Steel.

The present invention is concerned with the surface treatment of mild steel having low carbon content below 0.2%. We have found that the surface of such steel can be strengthened considerably by heating the steel in an atmosphere of ammonia at a temperature above 450 C. but below 590 C. for a period sufficiently long to form in the steel an upper surface layer consisting of nitrogen combined with iron to form iron nitride, and a lower surface layer comprising nitrogen in solid solution, and then quenching the steel to cool it rapidly. We find that any heating time in excess of 30 minutes produces a useful result, the optimum effect occurring after about 3 hours.

The lower temperature is the temperature at which sufiicient nascent nitrogen will be present in the ammonia to form the lower surface layer, which in a typical case contains about 7% by weight of nitrogen at 570 C. The upper temperature is chosen to be below the transition temperature of the steel. For steel having a carbon content below 0.2%, the transition temperature is normally in the region of 900 C. but, this temperature is lowered by the nitrogen to the value of 590 C. If the heating is carried out above 600 C. (allowing a 10 margin for safety), a transition occurs and on quenching the lower layer is converted to martensite. This procedure forms the subject matter of a further application filed with this application as a continuationin-part of application No. 214,818. The upper surface layers are subsantially the same Whether heating is carried out below 590 C. or above 600 C., and in each case improve seizure resistances, but the lower surface layers diifer. In the present application, the lower surface layer is very thick and may for example be 60 times thicker than the upper surface layer. The lower surface layer is also very hard, but nevertheless it is possible to work a component after the surface treatment. Where heating is carried out above 600 C., the lower surface layer is thinner and may be only about ten times as thick as the upper surface layer. However, the lower surface layer is so hard that a component cannot be worked after treatment.

The atmosphere in which heating takes place may consist of ammonia, or ammonia together with a hydrocarbon or other carbon-containing gas. Where a carboncontaining gas is employed, the upper surface layer will consist of compounds of iron, nitrogen and carbon. The amount of carbon-containing gas used depends on the particular upper surface layer required, but a figure of 20% by volume of carbon-containing gas is typical.

In one particular example it was required to treat the surface of a mild steel component containing up to 0.2%

carbon together with impurities which typically may include manganese, silicon, sulfur and phosphorus. The component was placed in a furnace at a temperature of 570 C. in an atmosphere of ammonia together with about 20% by volume of a carbon-containing gas. The com ponent was left in the furnace for 2 hours, and then quenched by immersing it in oil. The component was found to have an upper surface layer 0.0005 inch thick containing compounds of iron, carbon and nitrogen, and a lower layer 0.03 inch thick containing nitrogen in solid solution.

The present invention should not be confused with the well known nitriding process for steel. In all known nitriding processes, alloyed steel is used, that is to say the steel which is used has special components added to it to enable the nitriding to be carried out. Because of this, such steels are very expensive. The present invention applies only to mild steels, i.e., steels having a carbon content below 0.2% and the usual impurities, but no other elements. Such steels are considerably cheaper than alloyed steels and so the present process is a great step forward. It is true that the actual process is similar to known nitriding processes, but nevertheless it has never been proposed to apply such processes to mild steels, in spite of the fact that great economic benefits are obtained, and nitriding has been known for about 40 years.

Havin thus described our invention what we claim as new and desire to secure by Letters Patent is:

1. A method of treating the surface of mild steel which contains less than 0.2% carbon together with conventional impurities usually found in mild steel, comprising heating the steel for at least 30 minutes in an atmosphere which consists predominantly of ammonia but includes in addition a carbon containing gas, at a temperature above 450 C. but below 590 C., said heating forming in the steel a relatively thin upper surface layer in which nitrogen is combined with iron and a considerably thicker lower surface layer in which nitrogen is in solid solution, and then quenching the steel.

2. A method of manufacturing a steel component having a hardened surface, comprising the steps of forming the component in an unfinished condition from mild steel containing less than 0.2% carbon together with impurities usually found in mild steel, heating the component for at least 30 minutes in an atmosphere which consists predominantly of ammonia but includes in addition a carbon containing gas, at a temperature above 450 C. but below 590 C., said heating forming on the component a relatively thin upper surface layer in which nitrogen is combined with iron and a considerably thicker lower surface layer in which nitrogen is in solid solution, quenching the component and finally working the component to the desired shape.

References Cited by the Examiner UNITED STATES PATENTS DAVID L. RECK, Primary Examiner. 

1. A METHOD OF TREATING THE SURFACE OF MILD STEEL WHICH CONTAINS LESS THAN 0.2% CARBON TOGETHER WITH CONVENTIONAL IMPURITIES USUALLY FOUND IN MILD STEEL, COMPRISING HEATING THE STEEL FOR AT LEAST 30 MINUTES IN AN ATMOSPHERE WHICH CONSISTS PREDOMINANTLY OF AMMONIA BUT INCLUDES IN ADDITION A CARBON CONTAINING GAS, AT A TEMPERATURE ABOVE 450*C. BUT BELOW 590*C., SAID HEATING FORMING IN THE STEEL A RELATIVELY THIN UPPER SURFACE LAYER IN WHICH NITROGEN IS COMBINED WITH IRON AND A CONSIDERABLY THICKER LOWER SURFACE LAYER IN WHICH NITROGEN IS IN SOLID SOLUTION, AND THEN QUENCHING THE STEEL. 